1. Field of the Invention
The present invention relates to a transmission power control method for controlling a transmission power of a downlink common channel and a controller, in a communication system such as a CDMA (Code Division Multiple Access) mobile communication system.
2. Description of the Related Art
Conventionally, a base station transmits a downlink signal to mobile stations located in a cell covered by the base station using a downlink common channel in a mobile communication system such as a CDMA mobile communication system.
FIG. 1 is a schematic diagram for explaining a system of setting a transmission power in a conventional mobile communication system.
As shown in FIG. 1, all mobile stations located in a cell 4 covered by a base station 2 can receive downlink signals in the conventional mobile communication system. Therefore, the base station 2 transmits the downlink signals with a preset and fixed transmission power in accordance with a request from a radio network controller 3, so that a mobile station 1a located at the furthermost edge of the cell 4 can receive the downlink signals correctly.
However, the downlink common channel is transmitted at electric field strength, so that the mobile station 1a which is located at the furthermost edge in the cell 4 can receive the transmitted downlink common channel.
Accordingly, there is a problem in that the transmission power is in excess of that required for transmission to another mobile station 1b located in the same cell 4, which leads to a possibility of excessive reception quality for the mobile station 1b. 
Moreover, there is a problem in that unnecessary power consumption, occurrence of interference to other mobile stations, and deterioration in a downlink capacity may be caused, depending on a communication environment around the mobile stations 1a and 1b. 